Motion sensors for sensing changes in the state of motion of an object, including changes in position, velocity, acceleration or orientation, are well known in the art and encompass devices such as accelerometers, vibrometers, inclinometers and gyroscopes. Modern motion sensors are generally fabricated using micro electro-mechanical system (MEMS) technology and are used in a variety of motion sensing applications in fields such as inertial navigation, consumer electronics, vibration, structural, seismic and submarine monitoring, as well as automotive, healthcare, aerospace and military applications.
Among the different types of motion sensors, accelerometers are devices for measuring acceleration, typically by transducing the acceleration to be measured into a displacement of a proof mass connected to the casing or frame of the accelerometers. The behavior of the proof mass may be conceptually modeled as that of a damped mass attached to a spring. The displacement of the movable mass relative to the casing may be recorded by a displacement sensor, from which the acceleration acting on the casing may be derived. Measurements of the amplitude and frequency of the displacement of the proof mass can be performed using one of a number of transduction methods including capacitive, piezoresistive, piezoelectric, electromagnetic, thermal tunneling, and optical detection technologies.
Optical displacement sensors are particularly attractive as they offer potential for high-resolution and low-noise measurements exhibiting robustness to electromagnetic interference. Moreover, in some implementations, these sensors can operate without any electronic or electrical input/outputs, which may allow for remote operation in environments where electronic components cannot operate. Optical displacement sensors generally operate by modulating one or more characteristics of an optical beam, including power, phase, wavelength and polarization, in response to the displacement being measured. A variety of optical displacement sensors have been designed over the years, mostly involving optical fiber technology, and they include fiber Bragg gratings and Fabry-Perot interferometers. A drawback of most of these technologies is that a separate optical fiber is needed to sense along each sensing axis.
Another type of optical displacement sensor that is known in the prior art is a two-dimensional optical accelerometer based on a commercially available DVD optical pick-up head that measures the relative angle between a proof mass and a base, as described, for example, in Chu et al. “Two-dimensional optical accelerometer based on commercial DVD pick-up head”, Meas. Sci. Technol. vol. 18 (2007) p. 265-274. In this paper, a plane mirror is mounted on the proof mass and the proof mass tilts when it is accelerated. A laser beam is projected onto the plane mirror, reflected thereby, and focused on a four-quadrant photodiode. As known in the art, a quadrant photodiode is a type of optical quadrant sensors that can yield absolute position measurements along two orthogonal axes that intersect at the reference position of a light beam impinging onto the quadrant sensor. As the angle of the plane mirror changes in response to a displacement of the proof mass, the position of the beam moves across the surface of the photodiode. The four-quadrant photodiode converts the optical power of the incident focused laser beam into electrical signals. Deviations of the focused laser beam from the center of the four-quadrant photodiode produce corresponding changes in the magnitudes of the electrical signals outputted by the four quadrants of the photodiodes. The magnitudes of the electrical signals can then be used to determine the incident position of the focused laser beam on the four-quadrant photodiode and, in turn, the displacement of the proof mass and the acceleration of the base. However, such an accelerometer requires electrical access at the measurement point, which may be a drawback in some applications. For example, in oil and gas applications such as seismology and well-deviation monitoring, the increased weight and system complexity arising from the provision of electrical connections and wiring may be problematic and costly.
In light of the above, there remains a need in the art for an optically-based motion responsive sensor capable of providing all-optical sensing capabilities at the point of measurement, while also alleviating at least some of the drawbacks and limitations found in known motion sensors.